1. Field of the Invention
The present invention relates to optical fibers and, more particularly, to optical fiber sensors to measure deformation.
2. Background Art
Optical fiber systems offering sensing functions have been used in numerous applications. One known sensor technology for optical fibers is the Bragg grating sensor. The Bragg grating is a periodic variation of the refractive index along the fiber axis and it is photoimprinted in the core of the fiber by using UV light. The main property of a Bragg grating is that part of the light going along the fiber is reflected back by the grating. This process is wavelength selective and the wavelength with the highest reflectivity is called the Bragg wavelength (typically located at the center of the spectral response). Any temperature variation or strain applied to the optical fiber might result in a Bragg wavelength shift and/or spectral deformation, depending on how perturbations are applied to the fiber.
United States patent application Publication No. US 2004/0234218, by inventors Tao et al. discloses an optical fiber device that is applied for the measurement of multiple parameters, such as deformation. More specifically, the optical fiber device described in Publication No. US 2004/0234218 has at least two cores within a cladding, with the two cores respectively provided with Bragg grating sensors. The optical fiber of the Tao et al. reference describes a fiber configuration in which four cores, each with different characteristics, are provided (FIG. 1 of US 2004/0234218), with a central core, and three peripheral cores, at an equal radial distance from the central core and 120 degrees apart. Such an optical fiber is suitable for decoupling the effect of bending from the effect of temperature on the Bragg gratings of the fiber. However, an orientation of the fiber must be considered in such decoupling, but this publication provides no detail as to how the fiber may be oriented in a desired manner. Moreover, it is noted that in embodiments each fiber core in this publication has different characteristics (optical, thermal and mechanical) due to different chemical compositions.